The present invention relates to an erasing head which erases a magnetic record on a magnetic recording medium.
FIG. 7 shows a prior permanent magnetic type erasing head, which has a permanent magnet 3. The width of the magnet 3 is half of the slider plane 2 which contacts with a magnetic medium like magnetic tape which is subject to erasure, so that said magnet 3 contacts with the track which is subject to erasure, and another track of the magnetic tape contacts with the non magnetic plastics housing 1 so that said other track is not erased.
FIG. 8 is another prior erasing head, in which the permanent magnet 3A is mounted in the non-magnetic plastic housing 1A so that one of the poles (N or S) is disposed on the lower (or upper) half of the slider plane 2. Further, the magnetic material with high permeability 4 like permalloy is mounted along the boundary of the tracks to prevent cross erasing.
However, the erasing heads of FIGS. 7 and 8 have the disadvantage that the manufacturing cost is high, because there are many manufacturing steps. In manufacturing the erasing head of FIG. 7 or FIG. 8, the plastics housing 1, and the permanent magnet 3 are molded separately, then, that housing and magnet are adhered to each other. Further, the slider plane 2 must be polished to remove the step between the magnet and the housing plane Further, the high permeability material 4 must be mounted in case of FIG. 8.
In order to solve the above disadvantage, we considered an erasing head which has a permanent magnet material on the whole slider plane, but said permanent magnet material is magnetized only half of its width. However, we found that said type of erasing head has the problem of an undesired natural magnetization of the magnetic material.
Said natural magnetization is a small amount of magnetization which is generated during the manufacturing process of a permanent magnet, and said natural magnetization should be differentiated from the desired magnetization for providing a permanent magnet. Said natural magnetization is generated when a magnetic material is located in a weak magnetic flux at the temperature near the Curie temperature, or when said magnetic material is locally heated by polish process. The natural magnetization is practically not erased even when the material is heated higher than the Curie temperature, and weak magnetization remains. Said natural magnetization occurs not only in ferrite material, but also in a rare-earth magnet.
As mentioned above, a permanent magnet is inherently magnetized not only on the desired portion, but also on the undesired portion. Accordingly, when an erasing head with the double track width having half of its width magnetized is used for erasing a double track magnetic medium, the undesired natural magnetization on the track which is not desired to be erased is also magnetically effected (for instance, S/N ratio is deteriorated). Accordingly, an erasing head with a permanent magnet which has a double track width and is magnetized half of its width has not been practical.
Further, it should be noted that said undesired natural magnetization affects even the track which is subject to erasure. Since the erasing magnetization has the characteristics that the flux decreases along the running direction of a magnetic medium which is subject to erasure, the erasing flux is affected by the undesired natural magnetization on its neighboring track. In particular, the portion where the flux is weak on the erasing track is affected by the undesired natural magnetization. Therefore, the erasing operation on the erasing track is incomplete, and the S/N ratio is deteriorated when a signal is recorded on the erased track.
Another prior art is shown in Japanese patent laid open publication Nos. 80820/80, 42602/84, and the Japanese utility model laid open publication Nos. 151216/75 and 110718/82, in which a non magnetic-film is coated on magnetic pole portion of a permanent magnet However, those publications do not solve the problem of undesired natural magnetization on the neighboring track which is not erased.